1. Field of the invention
This invention relates to a method for producing a silicon carbide semiconductor device, and more particularly, to a method for producing a silicon carbide semiconductor device having a silicon carbide semiconductor layer, the conductivity type of which is controlled by ion implantation.
2. Description of the prior art
Semiconductor devices (which include diodes, transistors, integrated circuits, large-scale integrated circuits, light-emitting diodes, and charge-coupled devices) using silicon (Si) or a compound semiconductor such as gallium arsenide (GaAs) and gallium phosphide (GaP), have been widely used for practical applications in various fields of electronics.
Silicon carbide (SiC) is a semiconductor material with a wide energy gap of 2.2 to 3.3 electronvolts (eV), which is thermally, chemically and mechanically quite stable, and also has a great resistance to radiation damage. Furthermore, the saturation drift velocity of electrons in silicon carbide is greater than that in silicon (Si) and other semiconductor materials. The use of semiconductor devices made of conventional semiconductor materials such as silicon is difficult under severe conditions of high temperature, high output drive, high frequency operation, and radiation exposure. Therefore, semiconductor devices using silicon carbide are expected to have wide applications for devices which can be used under such conditions.
Despite these many advantages and capabilities, a silicon carbide semiconductor device has not yet been in actual use, because the technique still remains to be established for growing high quality silicon carbide single crystals having a large surface area with good reproducibility required for the commercial production thereof.
Conventional processes for preparing single-crystal substrates of silicon carbide on a laboratory scale include the so-called sublimation method (i.e., the Lely method) wherein silicon carbide powder is sublimed in a graphite crucible at 2,200.degree. C. to 2,600.degree. C. and recrystallized to obtain a silicon carbide single crystal, and the so-called epitaxial growth method wherein the silicon carbide single crystal obtained by the sublimation method is used as a substrate and a silicon carbide single-crystal layer is grown on the substrate by chemical vapor deposition (CVD) or liquid phase epitaxy (LPE), resulting in a silicon carbide single crystal, the size of which is sufficient to produce semiconductor device elements therefrom. Although a large number of crystals can be obtained by either the sublimation method or the epitaxial growth method, it is difficult to prepare large single crystals of silicon carbide and to control with high accuracy, the size and shape of single crystals of silicon carbide. Moreover, it is also difficult to control the polytype and the impurity concentration of these single crystals.
In recent years, the inventors have developed a process for growing large-sized high-quality single crystals of silicon carbide on a single-crystal substrate of silicon by the chemical vapor deposition (CVD) technique and filed a Japanese Patent Application No. 58-76842 (76842/1983) which corresponds to U.S. Pat. No. 4,623,425. This process (referred to as a successive two step CVD technique) includes growing a thin film of silicon carbide on a silicon substrate by the CVD technique at a low temperature and then growing a single-crystal film of silicon carbide on the said thin film by the CVD technique at a higher temperature. At the present time, this technique makes it possible to control the conductivity type, the impurity concentration, or the like of silicon carbide single crystals obtained by adding an appropriate amount of impurities during the growth of the single crystals. Therefore, this technique makes many contributions to the development of various semiconductor devices in which silicon carbide single crystals are used.
Moreover, it is disclosed that silicon carbide layers grown on silicon substrates or silicon carbide substrates can be doped during the growth by chemical vapor deposition or doped after the growth by ion implantation with boron (B) or aluminum (Al) ions as the p-type impurity or phosphorous (P) or nitrogen (N) ions as the n-type impurity (see, e.g., H. Kong, H. J. Kim, J. A. Edmond, J. W. Palmour, J. Ryu, C. H. Carter, Jr., J. T. Glass, and R. F. Davis, Mat. Res. Soc. Symp. Proc., Vol. 97 (1987), pp. 233-245).
However, because temperatures used for chemical vapor deposition are usually high, for example, silicon carbide semiconductor devices with a p-n junction, which are obtained using a chemical vapor deposition technique, will exhibit unsatisfactory electrical characteristics. Therefore, the above-mentioned method is not suitable for forming silicon carbide semiconductor layers to which a certain amount of impurities are added.
For the production of semiconductor devices using silicon (more particularly, those having a planar configuration), thermal impurity diffusion or ion implantation has been widely used as an essential process technique therefore.
However, thermal impurity diffusion is not suitable as a process technique for the production of silicon carbide semiconductor devices, because the diffusion constant of impurities in silicon carbide is quite small and because diffusion temperatures of 1,600.degree. C. or more are required for this technique.
Referring to ion implantation as a process technique, implantation of the group V element ions (e.g., nitrogen (N) or phosphorous (P) ions) is used for practical application. The silicon carbide layer in which these ions have been implanted can be electrically activated by thermal annealing. However, electrical activation for the group V element ions (i.e., ratio of the sheet carrier concentration to the dose of implanted ions) is as low as 80% or less, even when thermal annealing for the activation of implanted ions is conducted at high temperatures of 1,300.degree. C. or more. In cases where the group III element ions (e.g., boron (B) or aluminum (Al) ions) are implanted in a silicon carbide layer (for example, the inventors have devised a method for forming a high-resistance silicon carbide single-crystal layer by implanting the III group element ions, such as boron (B) ions, aluminum (Al) ions, or gallium (Ga) ions, and filed a Japanese Patent Application No. 1-75665 which corresponds to U.S. patent application Ser. No. 499,889), although the implanted impurity ions are distributed in the silicon carbide layer according to the view based on the theory, the silicon carbide layer is not activated electrically to a satisfactory extent.